Despite intensive research efforts, the cause of Anterior Cruciate Ligament (ACL) injury remains elusive, beyond the simple observation that women are at higher risk than men. We hypothesize that the etiology of ACL injury is multifactorial in nature, with neuromuscular risk factors playing a critical role. This hypothesis is suggested by biomechanical studies indicating that the increased risk in women may be largely due to neuromuscular risk factors, such as gender differences in muscle strength and human movement. These neuromuscular differences between women and men are particularly evident in the landing phase of a jump. We therefore propose poor technique when landing from a jump (jump-landing) as a useful marker of neuromuscular risk factors associated with both gender and ACL injury. A standardized tool for scoring poor jump-landing technique, the Landing Error Scoring System (LESS), has been developed. It has been validated and has good reliability. This study will utilize the LESS, 3-D motion analysis, and muscle strength testing to examine the association between neuromuscular risk factors and ACL injury. The study will also examine other risk factors (demographic, hormonal, and anatomical) identified in the literature as potentially associated with ACL injury, and will compare the prevalence and determinants of poor jump-landing technique between women and men. The study will use a prospective cohort design. It will enroll 4,800 male and female cadets at the three largest U.S. military academies over a 4 year period. Each study participant will undergo a baseline assessment that includes measurement of neuromuscular risk factors using motion analysis, muscle strength testing, and assessment of jumplanding technique using the LESS. The participants will be followed for up to 4 years with prospective identification and verification of ACL injuries. The subject recruitment and baseline testing procedures have been successfully piloted. This focus of this prospective cohort study is on neuromuscular risk factors for ACL injury, with an emphasis on neuromuscular risk factors that exhibit gender differences. The study will also investigate demographic, hormonal, and anatomical risk factors that have been advanced in the literature as causes of the excess of ACL injuries in women. Ultimately, the research will make a significant contribution to our understanding of the causes of ACL injury, with particular regard to those risk factors that appear to predispose women to ACL injury. Unlike some of the anatomical and hormonal risk factors discussed in the literature, neuromuscular risk factors are readily modifiable and are amenable to intervention.